1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines, for purging evaporative fuel generated in the fuel tank into the intake system of the engine, and more particularly to an evaporative fuel-processing system of this kind which is capable of performing a diagnosis of abnormality of its own operation.
2. Prior Art
There has been known an evaporative fuel-processing system for an internal combustion engine having a fuel tank, which comprises a canister communicating with a fuel tank, and a purge control valve arranged across a purging passage extending from the canister to the intake system of the engine, wherein evaporative fuel generated in the fuel tank is temporarily stored in the canister and then suitably purged into the intake system of the engine. To determine abnormality of the thus constructed evaporative fuel-processing system, an abnormality-determining method has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 4-362264, according to which the interior of the evaporative fuel-processing system is negatively pressurized, and then the purge control valve is closed, followed by determining a variation in the pressure within the evaporative fuel-processing system over a predetermined time period after the purge control valve is closed with the system negatively pressurized, to thereby determine whether or not there is an abnormality in the system, based on the determined variation.
However, according to the above proposed conventional method, a pressure sensor which detects pressure within the evaporative fuel-processing system is provided in a charging passage connecting between the fuel tank and the canister, and as a result, there can occur a pressure loss due to flow resistance of a portion of the charging passage extending between the pressure sensor and the fuel tank, so that a value of pressure detected by the pressure sensor (sensor output value) and the actual pressure within the fuel tank do not agree with each other, which may result in that the pressure within the fuel tank cannot be accurately reduced to a desired value.